The present invention generally relates to the field of induction heating. The present invention more particularly relates to an induction heating coil and method of using the induction heating coil to rapidly and uniformly heat a workpiece.
An induction heating apparatus generally includes an alternating current power supply that is connected to an induction coil. The induction coil can be a single turn coil, but typically includes a plurality of individual turns of electrically conductive wire or tubing. An induction coil including a plurality of individual turns is commonly known as a multi-turn coil.
Induction heating is performed by disposing a workpiece to be heated within the induction coil and passing an alternating electric current through the induction coil. As the alternating electric current is passed through the induction coil, the coil generates an electromagnetic field. The eddy currents induced in the workpiece by the electromagnetic field causes the workpiece to become heated.
Induction heating is commonly used to heat industrial gas cylinders. The optimum configuration for an induction heating coil powered from a single power supply would include a coil that has a configuration that generally matches the geometrical shape of the cylinder to be heated, while maintaining a satisfactory air gap between the inner surface of the coil and the outer surface of the workpiece. The use of such a coil to heat an industrial cylinder, however, results in the sides walls of the cylinder being heated much more rapidly than the domed end of the cylinder. This non-uniform heating of the cylindrical workpiece can be contributed to the fact the electromagnetic field flux of such a coil weakens as the side walls of the workpiece approaches the end of the coil.
If the power supply to the induction coil is terminated when the side walls of a cylindrical workpiece initially reaches a desired predetermined temperature, then the domed end of the workpiece will not reach the desired temperature. Additionally, if the induction coil continues to be energized by the power supply until the domed end of the cylindrical workpiece reaches the desired temperature, then the side walls of the workpiece will exceed the desired temperature and, therefore, will be too hot. Under both circumstances, the process will result in a workpiece that is not uniformly heated.
In addition, an ever present problem with the use of a multi-turn cylindrical induction coil is that the electromagnetic field produced by the induction coil varies along the length of the coil. Heating of a workpiece with a multi-turn cylindrical induction coil, therefore, often results in the non-uniform heating of a workpiece.
Many attempts have be made in the art to overcome the problems of nonuniform induction heating of workpieces. U.S. Pat. No. 2,422,417, for example, discloses an induction heating apparatus with a rotatable work conveyer to provide uniform heating of a workpiece.
U.S. Pat. No. 2,823,289 discloses an induction heating apparatus including a single induction heating coil and a sleeve or object-guiding insert that is placed into the induction heating coil.
U.S. Pat. No. 3,120,596 discloses an induction heating coil for even heating of elongated workpieces of differing lengths. The induction heating coil includes a first coil section that has a length that is adequate for even heating of the shortest workpieces, together with a group of axially spaced coil sections at the end of the first coil section.
U.S. Pat. No. 3,612,804 discloses an induction heating coil including an elongated coil having an elongated internal passageway through which members pass and an improved guide means extending between the entrance and exit of the passageway for guiding members through the coil. The improved guide means includes a contoured portion between the entrance and exit of the coil such that the members do not pass through the coil with their abutting ends in full contact with each other.
U.S. Pat. No. 3,725,630 discloses an inductive coil for heating a loop of conductive material. The inductive coil includes first and second concentric closed inductive loops of different sizes to receive a loop of conductive material between them. The inductive loop provides a structure wherein current will flow through the loops in series and in opposite directions to provide uniform heating of a closed loop of conductive material.
U.S. Pat. No. 4,538,279 discloses an induction coil in the form of a pancake coil for crucible-free zone melting of semiconductor crystal rods. The coil includes a primary winding surrounding a semiconductor rod to be remelted in the form of a ring through which cooling liquid flows, a secondary winding surrounding the primary winding and having a side facing the semiconductor rod and an energy concentrator lying in the plane of the primary winding.
U.S. Pat. No. 4,468,549 discloses an induction heater arrangement for forging bar stock. The heater arrangement includes an induction heating coil with a longitudinally extending axial passageway for receiving bar length to be heated. A reciprocal flux diverter is moved into the open end of the heating coil passageway to surround the residually heated terminal end of the bar length to divert the flux and prevent it from penetrating and heating the residually heated terminal end of the inserted bar.
Prior attempts at simultaneously heating the side walls and domed ends of an elongated industrial cylindrical workpiece to a predetermined uniform temperature have included the use of two independent power suppliesxe2x80x94one power supply for the side wall coil and one power supply for the domed end coil. The use of two independent power supplies was employed in an attempt to allow the characteristics of the dissimilar zones of the side walls and domed end to be matched.
Another attempt at simultaneously heating the side walls and domed ends of an elongated cylindrical workpiece to a predetermined uniform temperature included the use of a complex electronic power sharing arrangement that energizes each individual induction coil.
The efficiency of an induction heating device ultimately depends on the amount of electromagnetic energy generated by an induction coil that can be converted into heat energy in the workpiece. The prior art methods utilize more complex, more costly, but less efficient equipment for induction heating of workpieces. As such, there still remains a great need in the art for a simple and cost effective configuration for an induction heating coil and device for rapidly and uniformly heating workpieces. In particular, there is a great need for a simple and cost effective induction heating coil and device for rapidly, simultaneously and uniformly heating all portions of an elongated industrial cylinder workpiece, including the side walls and domed end portions of an industrial cylinder.
The present invention, in one embodiment, includes an induction heating coil comprising an elongated solenoid-type induction heating coil having opposite open ends and at least one pancake-type coil entirely positioned within a portion of said elongated solenoid-type induction heating coil, wherein said solenoid-type induction heating coil is in electrical connection with said pancake-type induction coil.
The present invention, in another embodiment, includes an induction heating device for heating a workpiece comprising: an induction heating coil comprising an elongated solenoid-type induction heating coil having opposite open ends and at least one pancake-type coil positioned entirely within a portion of said elongated solenoid-type induction heating coil, wherein said solenoid-type induction heating coil is in electrical connection with said pancake-type induction coil; and a high frequency power source in electrical connection with said solenoid-type coil and said pancake-type coil for supplying high frequency electric power to said coils.
The present invention, in another embodiment, includes a method of heating a workpiece comprising: providing an induction heating coil comprising an elongated solenoid-type induction heating coil having opposite open ends and at least one pancake-type coil positioned within a portion of said elongated solenoid-type induction heating coil, wherein said solenoid-type induction heating coil is in electrical connection with said pancake-type induction coil; positioning said workpiece to be heated in proximity to said pancake-type induction coil; and supplying said coils with a source of high frequency power to induce a current in the workpiece for a time sufficient to heat said workpiece to a desired temperature.